The standardized body, Motion Picture Experts Group (MPEG), discloses conventional data compression methods in their standards such as, for example, the MPEG-2 advanced audio coding (AAC) standard (see ISO/IEC 13818-7) and the MPEG-4 AAC standard (see ISO/IEC 14496-3). These standards are collectively referred to herein as the MPEG standard.
An audio encoder defined by the MPEG standard receives an input pulse code modulation (PCM) signal, converts it through a modified discrete cosine transform (MDCT) operation into frequency spectral data, and determines optimal scale factors for quanitizing the frequency spectral data using a rate-distortion control mechanism. The audio encoder further quantizes the frequency spectral data using the optimal scale factors, groups the resulting quantized spectral coefficients into scalefactor bands, and then subjects the grouped quantized coefficients to Huffman encoding.
According to the MPEG standard, the rate-distortion control mechanism operates iteratively to select scale factors that can produce spectral data satisfying two major requirements. Firstly, the quantization noise (audio quality) may not exceed allowed distortion that indicates the maximum amount of noise that can be injected into the spectral data without becoming audible. The allowed distortion is typically determined based on psychoacoustic modeling of human hearing. Secondly, the amount of used bits resulting from the Huffman encoding may not exceed an allowable amount of bits calculated from the bit rate specified upon encoding.
The rate-distortion control mechanism typically defines individual scale factors and a common scale factor. Individual scale factors vary for different scalefactor bands within the frame and a common scale factor is not changed within the frame. According to the MPEG standard, the rate-distortion control process iteratively increments an initial (the smallest possible) common scale factor to minimize the difference between the amount of used bits resulting from the Huffman encoding and the allowable amount of bits calculated from the bit rate specified upon encoding. Then, the rate-distortion control process checks the distortion of each individual scalefactor band and, if the allowed distortion is exceeded, amplifies the scalefactor bands, and calls the common scale factor loop again. This rate-distortion control process is reiterated until the noise of the quantized frequency spectrum becomes lower than the allowed distortion and the amount of bits required for quantization becomes lower than the allowable amount of bits.
The above-described conventional rate-distortion control process takes a large amount of computation because it has to process a wide range of possible scale factors. In addition, it lacks the ability to choose optimal scale factors when a low bit-rate (below 64 kbits/sec) is required.